<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
<script>
window.onload=function ()
{
	var oBtn=document.getElementById('btn1');
	
	oBtn.onclick=function ()
	{
		//1.创建Ajax对象
		if(window.XMLHttpRequest)
		{
			var oAjax=new XMLHttpRequest();
		}
		else
		{
			var oAjax=new ActiveXObject("Microsoft.XMLHTTP");
		}
		
		//2.连接服务器
		//open(方法, 文件名, 异步传输)
		oAjax.open('GET', 'a.txt', true);
		
		//3.发送请求
		oAjax.send();
		
		//4.接收返回
		oAjax.onreadystatechange=function ()
		{
			//oAjax.readyState	//浏览器和服务器，进行到哪一步了
			if(oAjax.readyState==4)	//读取完成
			{
				if(oAjax.status==200)	//成功
				{
					alert('成功：'+oAjax.responseText);
				}
				else
				{
					alert('失败:'+oAjax.status);
				}
			}
		};
	};
};
</script>
</head>

<body>
<input id="btn1" type="button" value="读取" />
</body>
</html>
